In a new attempt to eliminate the only FDA-approved class of weight-loss drugs, Public Citizen issued a petition to the U.S. Food and Drug Administration Thursday -- its second in five years -- calling for the agency to ban the class of drugs called orlistat, better known by the prescription brand names Xenical and Alli.

"These drugs have the potential to cause significant damage to multiple critical organs, yet they provide meager benefits in reducing weight loss in obese and overweight patients," said Dr. Sidney Wolfe, director of Public Citizen's Health Research Group.

An estimated 40 million people worldwide have taken either Xenical or Alli in the last decade. But sales have decreased substantially over the last decade. Alli sales dropped from $145 million in 2007 to $84 million by mid-2010, according to a report released by Public Citizen.

The drugs' maker, GlaxoSmithKline, announced Thursday it is trying to sell the brands.

Breakthrough Drug that Fizzled

In 1999, Xenical, sold by prescription, was touted as a breakthrough weight-loss drug. Alli was approved in 2007 as an over-the-counter drug. The FDA banned the two market-leading pills, Fen-phen and ephedra, because of potentially fatal side effects.

Studies suggest that taking Xenical or Alli would only help people lose 4 to 6 pounds more than they would with only diet and exercise.

"The benefit was never that great, but many people thought they'd take their chances," said Wolfe.

Drugs Block Fat Enzymes

The drugs work by blocking absorption of about a third of certain protein enzymes that enter the body. Instead, the fat passes through the body to the gastrointestinal tract until it is excreted. These medications also block fat-soluble vitamins including vitamins A, B, and K.

Some of the more common side effects of Alli or Xenical include diarrhea and stool leakage.

However, according to Dr. Donald Hensrud, associate professor of nutrition and preventive medicine at the Mayo Clinic in Rochester, Minn., the longer a patient stays on the medication, the more likely these symptoms will go away.

"The chances of having those side effects are a bit overrated," said Hensrud. "Only about 20 percent of people get those side effects, and in most people it lasts for less than a week."

More severe side effects reported to the FDA include liver disease, pancreatitis, and kidney stones. The FDA received 47 reports of acute pancreatitis and 73 cases of kidney stones attributed to orlistats.

While many experts say these side effects are rare, it's not clear what segment of people who use the drug may experience these severe conditions.

Monique Paulwell of Bowie, Md. said she only took Alli four times before she began feeling fatigue, loss of appetite, a nagging headache and jaundice.

"After a battery of tests, [doctors] said there had been an attack on my liver," said Paulwell. "By the time I was admitted to the hospital, I had 48 hours to live. It was that serious."

Paulwell said she thought taking Alli would help her lose a few pounds and maybe boost her acting career. Instead, she had a near-fatal experience her doctors told her was caused by the drug. She said she needed a liver transplant to save her life.

"What I knew is that it was FDA approved, it was safe for me to use," she said. "But what happened was I started to feel ill almost immediately after starting to use it."

Public Citizen Urges Ban of Alli and Xenical Weight-Loss Drugs

"GSK is committed to ensuring that consumers and physicians understand the safety profile of orlistat and alli," it said. "GSK continues to proactively monitor for the safety of orlistat and share all relevant information with regulatory agencies."

In fact, those who are overweight are already at a higher risk for liver disease, Hensrud said.

"So it might not be the medication," he said. "It might just be the people who would probably take these medications would already experience these effects. The data is not consistent."

The FDA did not accept other popular weight-loss drugs such as Qnexa and Meridia in 2010. Still, the pharmaceutical advisory firm Decision Resources says it estimates that the weight-loss drug market will increase more than nine-fold over the next decade worldwide -- from $477 million in 2009 to more than $4.3 billion in 2019.

Market May Do What FDA Does Not

The FDA rejected Public Citizen's first petition in April 2006, which cited a preliminary study in rats that suggested orlistat caused pre-cancerous lesions in the colon.

It's unclear whether the FDA will accept Public Citizen's new petition. And in the end, some experts say that the market, rather than the FDA, may end up deciding the fate of the weight-loss drugs.

"It seems like Alli is going to wither away on its own anyway. Its market has already decreased considerably," said Keith Ayoob, a registered dietician and director of the nutrition clinic at the Rose R. Kennedy Center at the Albert Einstein College of Medicine. "It wouldn't surprise me if the FDA allowed it to continue with restriction without pulling it."